This invention describes new and useful methods for the degradation and removal of lignin from lignocellulosic materials; to delignify and brighten cellulosic pulps in bleaching; to produce cellulosic pulps for use in paper and paperboard manufacture and the manufacture of regenerated cellulose products; to enhance the strength, optical properties and other properties of recycled cellulosic fibers; to produce fodders having increased digestibility to ruminants; and to produce any other product in which the: degradation of or the removal of lignin from a lignocellulosic material produces beneficial results.
Lignocellulosic materials is a broad term that can be applied to a wide range of materials generally derived from plants or other organic sources. A primary example of such a material is wood. As is generally true for lignocellulosics, wood is composed of two main parts-a fibrous carbohydrate or cellulosic portion, and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin. A major economic use of wood is derived from the conversion of the wood into a form suitable for the manufacture of paper, paperboard, and other related products. Despite the economic importance of the industry that is founded on the conversion of the lignocellulosic content of wood into paper, the basic processes for delignifying wood, or significantly reducing its lignin content, for papermaking apply to all processes for which the purpose is to enhance the value or utility of a lignocellulosic material by modification of or reduction of its lignin content.
For use in paper-making processes, wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited on a screen to form a sheet. The methods used to accomplish this pulping usually involve either a physical or chemical treatment of the wood or perhaps some combination of the two processes, to alter its physical and chemical form to give the desired paper properties.
Current industrial processes for pulping wood and other sources of lignocellulosic material such as annual plants, and for bleaching the resultant pulp, have evolved slowly over many decades. Although these processes are quite complex and energy-intensive, they are relatively efficient. Their major disadvantage is that the chemical processes involved have the capacity to create a negative impact on the environment. Even the best of current technology is unable to completely suppress the odors emitted by pulp mills, or to completely eliminate the emission of chlorinated organic compounds from waste treatment plants associated with pulp mill bleach plants. The discovery of new methods for more easily or more effectively modifying or delignifying wood such as those disclosed herein can lead to the development of new, more efficient, less environmentally troublesome pulping and bleaching processes.
Pulping is achieved by chemical or mechanical means or combinations of the two. In mechanical pulping, the original constituents of the fibrous material are essentially unchanged, except for the removal of water soluble constituents. Chemical pulping, in contrast, has as its purpose the selective removal of the fiber-bonding lignin to a varying degree, while minimizing the degradation and dissolution of the hemicelluloses and cellulose. If the ultimate purpose of the pulp is the preparation of white papers, the purification process begun through initial pulping is continued in subsequent bleaching steps. The bleaching process can result not only in a brightening of the resulting pulp, but also a further reduction in the lignin content of the pulp. The properties of the end products of the pulping/bleaching process such as, for example, papers and paperboards, will be determined largely by the properties of the pulps that are used in their manufacture. The properties of the pulps, in turn, are determined by the particular pulping processes employed, as well as the identity of the wood species or non-wood plant fiber lignocellulosic used as the raw material for the pulp.
A pulp produced solely by chemical methods is referred to as a full chemical pulp. In practice, chemical pulping methods are successful in removing most of the lignin; they also degrade a certain amount of the hemicellulose and cellulose so that the yield of pulp is low relative to mechanical pulping, usually between 40 and 50% of the original wood substance, with a residual lignin content on the order of 3-5%. These pulps can be characterized as high strength pulps, although their production can be costly both in terms of the consumption of chemicals in the process, as well as the loss of hemicellulose and cellulose content from the starting materials.
In typical chemical pulping, wood physically reduced to a chip form is cooked with the appropriate chemicals in an aqueous solution, generally at elevated temperature and pressure. The energy and other process costs associated with reaction processes at elevated temperatures and pressures constitute significant disadvantages for conventional pulping processes. The two principal methods are the (alkaline) kraft process and the (acidic) sulfite process. The kraft process has come to occupy the dominant position because of advantages in chemical recovery and pulp strength. The sulfite process was more common up to 1930, before the advent of the widespread use of the kraft process, although its use has increased somewhat in recent years.
The kraft process involves cooking wood chips in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S). The alkaline attack causes a breaking of the lignin molecule into smaller segments whose sodium salts are soluble in the cooking liquor. Kraft pulps produce strong paper products ("kraft" is the German word for strength), but the unbleached pulp is characterized by a dark brown color. The kraft process is associated with malodorous gases, principally organic mercaptans and sulfides, which cause environmental concern, to which anyone who has been in the olfactory proximity of a kraft pulp mill can attest.
The kraft process evolved over 100 years ago from soda cooking (which utilizes only sodium hydroxide as the active chemical), when Carl S. Dahl, a German chemist, introduced sodium sulfate into the chemical cooking system as a makeup chemical. Actual conversion to sodium sulfide (Na.sub.2 S) in the resultant cooking liquor produced a dramatic improvement in reaction kinetics and pulp properties when cooking softwoods. The fact that sodium sulfate is commonly used as a makeup chemical is the reason that the kraft process is sometimes called the "sulfate process". The process uses the combination of sodium hydroxide and sodium sulfide at a pH in excess of 12, at 160-180.degree. C. (320.degree.-356.degree. F.), corresponding to about 800 kPa (120 psi) steam pressure, for 0.5-3 hours to degrade and dissolve much of the lignin of the wood fibers. The comparative strength of the resulting pulp arises from the use of an alkaline sulfide solution and the shorter cooking times which, in turn, lead to less cellulose degradation. Despite a certain number of distinct disadvantages, not the least of which are the energy costs imposed by typical reaction conditions, about 75-80% of U.S. virgin pulp is produced by this process.
In the alternative sulfite process, a mixture of sulfurous acid (H.sub.2 SO.sub.3) and bisulfite ion (HSO.sub.3 -) is used to attack and solubilize the lignin component of the lignocellulosic starting material. Here, the mechanism of chemical attack removes the lignin as salts of lignosulfonic acid, and the molecular structure, although fragmented, is left largely intact. The cations for the bisulfite can be calcium, magnesium, sodium, or ammonium. Sulfite pulping can be carried out over a wide range of pH. "Acid sulfite" denotes pulping with an excess of free sulfurous acid (pH 1-2), while "bisulfite" cooks are carried out under less acidic conditions (pH 3-5).
Sulfite pulps are lighter in color than kraft pulps and can be bleached more easily, but the paper sheets are weaker than equivalent kraft sheets. The sulfite process works well for such softwoods as spruce, fir and hemlock, and such hardwoods as poplar and eucalyptus; but resinous softwoods and tannin-containing hardwoods are more difficult to handle. This sensitivity to wood species, along with the weaker pulp strength and the greater difficulty in chemical recovery, are the major reasons for the decline of sulfite pulping relative to kraft. The trend towards whole tree chipping puts sulfite at a further disadvantage because of its intolerance to bark.
Although all delignification or chemical pulping processes have as their desired end result the significant reduction of the lignin content of the starting lignocellulosic material, the characteristics of the individual processes chosen to achieve that end bear considerably on the properties of the resulting products manufactured from that pulp. In general, although the chemical goal of pulping or delignification processes is the separation of the fibrous carbohydrate content of the lignocellulosic material from the lignin content, it is not always possible or even desirous to remove the entire lignin component from the lignocellulosic starting material. The extent to which any chemical pulping process is capable of degrading and solubilizing the lignin component of a lignocellulosic material while minimizing the accompanying degradation of cellulose and hemicellulose is referred to as the "selectivity" of the process.
Delignification selectivity is an important consideration during pulping and bleaching operations where it is desired to maximize removal of the lignin while retaining as much cellulose and hemicellulose as possible. One way of defining delignification selectivity in a quantitative fashion is as the ratio of lignin removal to carbohydrate removal during the delignification process. Although this ratio is seldom measured directly, it is measured in a relative manner by yield versus Kappa Number plots. Although the slope of two plots corresponding to two different pulping or delignification processes may be the same, the process which produces a higher yield, as measured by the amount of pulp in comparison to the amount of the starting material, for the same degree of delignification is considered to be the more selective process. A high selectivity alone, however, does not mean that pulp "A" is better than "B" since such plots do not indicate the strength or the viscosity of the pulp. For example, acid sulfite pulping is, by this definition more selective than kraft pulping; however, acid sulfite pulp is weaker than kraft pulp because the cellulose fibers are weaker due to acid hydrolysis.
Another way of defining selectivity is as the viscosity of the pulp at a given low lignin content. This is usually done by plotting pulp viscosity versus Kappa Number and comparing the viscosities of the pulps at a selected Kappa Number. The higher the viscosity, the more selective the delignification, or pulping process. In general, for a given process, the higher the viscosity the stronger the pulp. This sometimes does not apply when comparing pulps produced by different processes. For example, for a given low lignin content, acid sulfite pulp will be higher in yield and in pulp viscosity than kraft pulp; however, the kraft pulp will have higher strength properties.
In the sulfite process, sulfonation and acid hydrolysis contribute to delignification, and acid hydrolysis to carbohydrate degradation and dissolution. In the kraft process, mercaptation (sulfidation) and alkaline hydrolysis contribute to delignification, and alkaline peeling and hydrolysis to the carbohydrate degradation. The delignification proceeds more rapidly in the sulfite cook than in the kraft cook, and lower temperatures can therefore be used in the former, which is fortunate because the hydrolysis of the glycosidic bonds of the carbohydrates occurs much more rapidly in acidic than in alkaline medium. Alkaline peeling reactions, on the other hand, require lower temperature than the alkaline delignification, and they unavoidably decrease the carbohydrate yield, to a degree which depends on both chemical and physical changes in their structure. Accessibility phenomena improve the selectivity of lignin removal, partly because in the early stages of the cook the morphological structure protects the carbohydrates from being attacked by the pulping chemicals, especially in the sulfite cook, and partly because some of the hemicelluloses are capable of rearrangements to a more ordered and less accessible structure during the cook. The net result of all these phenomena is that softwood pulp yields at a certain degree of delignification are about 3-5% of the wood higher for the sulfite than for the kraft process, whereas hardwood pulp yield are fairly similar.
The methods described above for the delignification or pulping of lignocellulosic materials, although each possess certain practical advantages, can all be characterized as being hampered by significant disadvantages. Thus, there exists a need for delignification or pulping processes which are advantageous economically, either in terms of cellulosic yield of the process or in terms of the chemical or process technology costs of the method; which are environmentally benign; which produce delignified materials of superior properties; and which are applicable to a wide variety of lignocellulosic materials. Such processes, as exemplified by the invention disclosed herein, have the added advantage of wide applicability well beyond the area of pulping.